The invention relates to the heat treatment of tungsten filaments would on molybdenum cores, wherein the spirals are drawn through a humid hydrogen atmosphere of temperature range about 1300.degree. C., then through a similar, but dry atmosphere of temperature range about 1700.degree. C. to 1850.degree. C. The invention refers also to an equipment for accomplishing the proposed method, the equipment formed of two heating tubes made of high-melting metal, an inflection wheel, a spiral winding/unwinding device, inlet means for introducing gas and cooling, current leads, as well as a temperature sensing and controlling unit.
In the technology of manufacturing light sources the step of preparing filaments from spirally coiled elements covers also the important process of heating, i.e. heat treatment of secondary tungsten coils wound over molybdenum core wires. The heating technology has two important phases, namely the degraphitizing heat treatment (for purifying) and the fixing of the filament on the molybdenum core wire (heat treatment for fixing).
According to prior art practice, purifying heat treatment is performed in a humid hydrogen atmosphere in the temperature range of about 1100.degree. C. to about 1300.degree. C., in the course of which, by the oxygen content of water vapour, the graphite used as lubricant in wire-drawing is burnt off. The fixing heat treatment applied after the filament-shaping operation for fixing the geometrical shape of the filament takes place at a temperature of about 1600.degree. C. in dry hydrogen.
During the above mentioned operations the drawing speed should be such as to provide for a duration of approximately 20 seconds for the heat treatment (i.e. the time when each individual element of the filament passes through the heating space). This results, however, in an extremely low drawing speed (0.01 m/s in a heating space of 200 mm length) and consequently in a very low productivity.
The coiled filament may be kept at a temperature of about 1600.degree. C. for a more prolonged time without becoming brittle, whereas above this temperature, with heat-treating times lying in the order of magnitude of a minute, primary recrystallization takes place in the tungsten filament. the filament becomes brittle and unsuitable for being installed. Moreover, above this temperature the molybdenum core also becomes brittle. On the other hand, the heat treatment can only be considered successful is neither the molybdenum core, nor the tungsten filament become brittle, and after withdrawal of the core, the filament retains its shape.
The disadvantage of the methods of the art described above is the high energy demand (in electric power and hydrogen) and relatively low speed of performance, i.e. the unsatisfactory productivity.